Recently, it has been strongly desired for electrical devices including a solenoid valve, a motor, a power supply circuit, or the like to have reduced size, increased efficiency, and increased output. Increasing the operating frequency of these electrical devices is effective in meeting these requirements. The operating frequency of solenoid valves, motors, and the like has been increased on the order of several hundreds of hertz to several kilohertz, and the operating frequency of power supply circuits has been increased on the order of several tens of kilohertz to several hundreds of kilohertz.
Hitherto, electrical devices such as a solenoid valve and a motor are usually operated at a frequency of several hundreds of hertz or lower, and an electrical steel sheet, which is advantageous in that it provides a low core loss, has been used for the material of an iron core of such electrical devices. The core loss of magnetic core materials is broadly divided into hysteresis loss and eddy-current loss. The above-described electrical steel sheet is produced by preparing sheets made of an iron-silicon alloy having a relatively low coercive force, performing an insulation treatment on the surfaces of the sheets, and then laminating the sheets. Such an electrical steel sheet is known as a material particularly having a low hysteresis loss. The eddy-current loss is proportional to the second power of the operating frequency, whereas the hysteresis loss is proportional to the operating frequency. Therefore, when the operating frequency is a band of several hundreds of hertz or lower, the hysteresis loss is dominant. The use of an electrical steel sheet, which particularly has a low hysteresis loss, is effective in this frequency band.
However, since the eddy-current loss is dominant in an operating frequency band of several kilohertz, an alternative material of an iron core replacing the electrical steel sheet is necessary. In such a case, a dust core and a soft ferrite magnetic core, which exhibit relatively satisfactory low-eddy-current loss characteristics, are effectively used. Dust cores are produced using a powdery soft magnetic material such as iron, an iron-silicon alloy, a Sendust alloy, a permalloy, or an iron-based amorphous alloy. More specifically, dust cores are produced as follows: A binder having an excellent insulating property is mixed with the soft magnetic material, or an insulation treatment is performed on the surface of the powder. The material thus prepared is then molded under pressure.
On the other hand, the soft ferrite magnetic core is known as a particularly excellent low-eddy-current loss material because the material itself has a high electric resistance. However, since the use of a soft ferrite decreases the saturation flux density, it is difficult to achieve a high output. The dust core is advantageous from this standpoint because a soft magnetic material having a high saturation flux density is used as a main component.
In a production process of a dust core, pressure molding is performed, and deformation during the pressure molding causes distortion of the powder. Consequently, coercive force is increased, resulting in an increase in the hysteresis loss of the dust core. Therefore, when the dust core is used as the material of an iron core, after a compact is prepared by pressure molding, a process of removing the distortion must be performed.
An effective process of removing such distortion is thermal annealing of the compact. When the temperature during this heat treatment is set to a high value, the effect of distortion removal is increased, thereby reducing the hysteresis loss. However, when the temperature during the heat treatment is set to an excessively high value, an insulating binder or an insulating coating constituting the soft magnetic material is decomposed or degraded, resulting in an increase in the eddy-current loss. Therefore, the heat treatment is inevitably performed only in a temperature range that does not cause such a problem. Accordingly, improving heat resistance of the insulating binder or the insulating coating constituting the soft magnetic material is important in order to decrease the core loss of the dust core.
A known typical dust core is produced by adding about 0.05 to 0.5 mass percent of a resin to a pure iron powder having a phosphate coating serving as an insulating coating, molding the powder under heating, and then performing thermal annealing for removing distortion. In this example, the temperature during the heat treatment is in the range of about 200° C. to 500° C., which is the thermal decomposition temperature of the insulating coating. In this case, however, the temperature during the heat treatment is low, and thus, a satisfactory effect of distortion removal cannot be achieved.
Japanese Unexamined Patent Application Publication No. 2003-303711 (Patent Reference 1) discloses an iron-based powder having a heat-resistant insulating coating with which insulation is not broken during annealing for reducing hysteresis loss, and a dust core including the iron-based powder. In the iron-based powder disclosed in Patent Reference 1, the surface of the powder containing iron as a main component is covered with a coating containing a silicone resin and a pigment. More preferably, a coating containing a silicon compound or the like is provided as an underlayer of the coating containing a silicone resin and a pigment. The pigment is preferably a powder having an average particle diameter, which is specified as D50, of 40 nm or less.
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2003-303711